The present invention relates to a color cathode-ray tube. In particular, the present invention relates to a color cathode-ray tube having a shadow mask with tension applied thereto in one direction.
In a color cathode-ray tube, an electron beam emitted from an electron gun illuminates a phosphor screen formed on an inner surface of a face panel, whereby a desired image is displayed. A shadow mask that functions as a color selection electrode is provided on the electron gun side of the phosphor screen and spaced at a predetermined distance. In the shadow mask, a number of substantially rectangular (slot-shaped) apertures (electron beam apertures) are arranged so that an electron beam collides with a phosphor at a predetermined position.
An electron beam emitted from the electron gun is deflected by a deflecting apparatus and passes through a predetermined apertures of the shadow mask to illuminate a phosphor at a predetermined position, whereby a satisfactory color image is displayed. The phenomenon in which an electron beam illuminates a phosphor different from a desired phosphor is called xe2x80x9cmislandingxe2x80x9d. When the mislanding occurs, deterioration of an image quality called a xe2x80x9ccolor shiftxe2x80x9d is caused.
The mislanding is caused by various factors, and various countermeasures are adopted in accordance with the respective generation factors.
A first generation factor of the mislanding is doming. The doming refers to a phenomenon in which the shadow mask is heated when an electron beam passes through an aperture to cause thermal expansion of the shadow mask. Because of this, an aperture position is changed and an electron beam passing through the aperture does not correctly illuminate a phosphor at a predetermined position, thereby causing the mislanding. In order to prevent this, the shadow mask is previously provided with tension so as to absorb thermal expansion caused by an increase in temperature, and the shadow mask in this state is stretched and held on a frame. Because of such stretch holding, even if the temperature of the shadow mask is increased, a relative positional shift between the aperture of the shadow mask and the phosphor stripe formed on the phosphor screen can be reduced.
A second generation factor of the mislanding is an external magnetic field such as a geomagnetism. When an external magnetic field acts on an electron beam, the track of the electron beam is bent to cause the mislanding. The direction of an external magnetic field is varied depending upon the setting direction of a color cathode-ray tube, and the magnitude thereof is varied depending upon the location of the color cathode-ray tube. Therefore, in order to display an image stably at all times irrespective of the setting direction and the location of the color cathode-ray tube, it is required to shield an electron beam from an external magnetic field. For this purpose, in general, an internal magnetic shield is placed between the frame on which the shadow mask is stretched and the deflecting apparatus, and the internal magnetic shield, the frame, and the shadow mask are made of materials with satisfactory magnetic permeability. Because of this, an external magnetic field is absorbed by the internal magnetic shield, the frame and the shadow mask to pass to the inside of the materials thereof. Thus, the action of an external magnetic field on an electron beam can be reduced.
The material for the shadow mask is selected considering the above-mentioned thermal and magnetic characteristics and cost. In general, a Fexe2x80x94Ni alloy (e.g., Invar material) or an iron (Fe) material (e.g., mild steel) is used. Among them, the Fexe2x80x94Ni alloy is more expensive than the iron (Fe) material. However, the Fexe2x80x94Ni alloy has a very small thermal expansion coefficient, so that it is effective for preventing the occurrence of the doming.
Furthermore, JP 10(1998)-302664 A describes the following: in the case where an aperture grill as a color selection electrode with tension applied thereto in one direction is stretched on a frame, the shape of the frame is designed so as to suppress a displacement amount in a tube axis direction of the aperture grill caused by thermal expansion, and simultaneously, the thickness of the frame in the tube axis direction is decreased to enhance a magnetic shield effect of an internal magnetic shield, whereby the mislanding of an electron beam can be reduced.
However, in a conventional color cathode-ray tube in which a shadow mask with tension applied thereto in one direction is stretched, the amount of mislanding of an electron beam caused by an external magnetic field such as a geomagnetism still may not be reduced sufficiently.
As a result of the earnest study of the cause for the above, it was found that the stress, which is applied to the shadow mask and the frame under the condition that the shadow mask is stretched, changes the magnetic characteristics of the shadow mask and the frame.
From the above point of view, it is an object of the present invention to provide a color cathode-ray tube in which the mislanding of an electron beam is unlikely to be caused by an external magnetic field, and consequently, a satisfactory color image display can be obtained.
In order to achieve the above-mentioned object, the present invention has the following configuration.
A first color cathode-ray tube of the present invention includes: a shadow mask on which a number of apertures for allowing an electron beam to pass through are formed; and a frame for stretching and holding the shadow mask with tension applied thereto in one direction, wherein the shadow mask is made of an Invar material, and a stress generated in the shadow mask by the tension is 32 MPa or more in a range of xc2x120 mm with respect to a center position of the shadow mask in a direction orthogonal to a direction in which the tension is applied, and is 26 MPa or more outside the range.
By setting a stress distribution in a direction orthogonal to a direction in which the tension of the shadow mask is applied as described above, a magnetic shield effect can be enhanced while a frame weight is suppressed. Therefore, a color cathode-ray tube can be provided in which mislanding and a color shift caused by the mislanding are unlikely to occur, the mislanding being caused by a change of a track of an electron beam due to the influence of an external magnetic field such as a geomagnetism that varies depending upon the direction of a screen and a geographic area of use. As a result, a satisfactory color image display can be obtained. Furthermore, by producing the shadow mask using an Invar material, mislanding of an electron beam caused by heating during use can be reduced.
A second color cathode-ray tube of the present invention includes: a shadow mask on which a number of apertures for allowing an electron beam to pass through are formed; and a frame for stretching and holding the shadow mask with tension applied thereto in one direction, wherein the shadow mask is made of a Fexe2x80x94Ni alloy, and the shadow mask is stretched and held by an average tensile stress of 29 MPa or more.
By setting an average tensile stress while the shadow mask is stretched at 29 MPa or more, a color cathode-ray tube can be provided in which mislanding of an electron beam caused by an external magnetic field is unlikely to occur, and as a result, a satisfactory color image display can be obtained. Furthermore, by producing the shadow mask using a Fexe2x80x94Ni alloy, mislanding of an electron beam by heating during use can be reduced.
In the above-mentioned second color cathode-ray tube, it is preferable that the shadow mask contains 36.1xc2x10.3% Ni (nickel). Because of this, mislanding caused by heating during use further can be reduced.
Furthermore, in the second color cathode-ray tube, it is preferable that a member constituting a longitudinal side of the frame is made of a Fexe2x80x94Ni alloy. By configuring the longitudinal side member, which directly stretches and holds the shadow mask, using the same kind of material as that of the shadow mask, the breakage and creases of the shadow mask caused by the difference in thermal expansion coefficient between the longitudinal side member and the shadow mask can be reduced.
Furthermore, in the above-mentioned second color cathode-ray tube, it is preferable that a member constituting the longitudinal side of the frame contains 36.1xc2x10.3% Ni (nickel). Because of this, the thermal expansion of the longitudinal side member caused by an increase in temperature during use can be reduced, so that the breakage and creases of the shadow mask and the displacement of apertures can be suppressed.
In the above-mentioned first and second color cathode-ray tubes, it is preferable that an average tensile stress of the shadow mask is 90 MPa or less. Because of this, an increase in weight and cost caused by an increase in tensile stress can be minimized. Furthermore, the vibration damping property of the shadow mask can be prevented from being degraded.
In the above-mentioned first and second color cathode-ray tubes, it is preferable that a total defection angle is 115xc2x0 or more. When the present invention is applied to a color picture tube with a total deflection angle of 115xc2x0 or more, the effects of the present invention are exhibited more remarkably.